🔍 Key Concepts

• Elongation: the angular distance between a planet and the Sun as seen from Earth • How inner planets (Mercury, Venus) show many phases vs how outer planets (like Mars) appear • What conjunction and opposition mean in terms of Sun–Earth–planet alignment

💭 Think About

• Think about where Mars is relative to Earth and the Sun when it is directly behind or directly opposite the Sun from our point of view • Consider which kinds of phases depend on a wide range of elongation angles, and which are only possible when the elongation is very small or very large • Ask yourself: when would we see the fully illuminated side of Mars, and when would the illuminated side be mostly turned away from us?

✅ Before You Answer

• Be clear on the definition of 0° elongation (planet in line with the Sun as seen from Earth) vs large elongation angles • Recall that only inferior planets (inside Earth’s orbit) show the complete set of crescent-to-full phases • Verify which of the options describe a situation where Mars, as an outer planet, would show one of its two observable phases